1. Field of the Invention
The present general inventive concept most closely relates to image processing, and more particularly, to an image processing method and an image processing apparatus, which convert a binary image having low resolution into a grayscale image having higher resolution.
2. Description of the Related Art
In general, performance of an image forming apparatus, such as a printer, a multifunction copier, and so on, is determined by such factors as print speed and image quality. Factors affecting the print speed include print resolution, print data transmission time from a host apparatus, such as a computer system, to the image forming apparatus, print data processing time in either or both of the host apparatus and the image forming apparatus, and printing time of a printer engine in the image forming apparatus. Historically, printing speed has been dominated by certain mechanical limitations of the printer engine. In recent years, however, with improvements to increase the speed of the printer engine, the print data transmission time and the print data processing time have dominated the overall time to print.
The print data transmission time and the print data processing time are determined by a data exchange system between the host apparatus and the image forming apparatus. For example, if a printer driver executing in the host apparatus employs a graphics device interface (GDI) system that performs color matching, image rendering, etc., print data may be compressed by a compression algorithm in the host apparatus, such as Joint Bi-level Image Expert Group (JBIG), to reduce transmission time for the data to traverse the distance from the host apparatus to the image forming apparatus. The transmitted print data are then decompressed and printed in the image forming apparatus. On the other hand, if the printer driver employs a page description language (PDL) system, the print data are transmitted as, for example, descriptors that are processed entirely in the image forming apparatus to render the image, unlike the GDI system.
FIG. 1 is a flow chart illustrating an example of a conventional process of transmitting print data from a host apparatus to an image forming apparatus. Referring to FIG. 1, at operation S1 halftoning is performed in the host apparatus on an 8-bit grayscale image having a resolution of 200×200 pixels and the grayscale image is converted into a one-bit binary image having resolution of 200×200 pixels. Next, JBIG compression is performed on the resultant binary image at operation S2 and the compressed binary image is transmitted from the host apparatus to an image forming apparatus at operation S3. At operation S4, JBIG decompression is performed on the binary image transmitted to the image forming apparatus. In this case, a high-capacity page memory is required to store the 200×200 one-bit image. Additionally, a large quantity of data must be transmitted since the compressed binary image also has a large quantity of data. If a page memory has the capacity to store an image transmitted from the host apparatus to the image forming apparatus, that is, there is a large quantity of transmission data, a long time is required to transmit the data to fill the page memory, prior to which no printing is performed.
FIGS. 2A and 2B are flow charts illustrating other examples of conventional processes to print data from a host apparatus to an image forming apparatus. FIG. 2A illustrates a case where the amount of data of an input image (hereinafter also referred to as “information quantity”) is reduced. In this example, operations S11 to S14 are similar to the operations S1 to S4 in the example of FIG. 1. The example of FIG. 2A additionally includes operation S15, where the 200×200 one-bit binary image decompressed in operation S14 is converted into a 200×200 8-bit grayscale image. Consequently, operation S15 increases the information quantity.
FIG. 2B illustrates a case where the size of an input image is reduced. In the example of FIG. 2B, operations S21 to S24 are similar to operations S11 to S14 in the example of FIG. 2A. The example of FIG. 2B additionally includes operation S20 where the size of an input image is reduced from 200×200 pixels to 100×100 pixels. The process of FIG. 2B further includes operation S25 to increase the spatial resolution of the 100×100 one-bit binary image, decompressed in operation S24, to produce a 200×200 one-bit binary image. The operation S25 expands the size of the image using an interpolation method or the like.
Using the above-described processes to reduce the information quantity or the size of the image prior to the transmission of the print data may result in a shortened transmission time. Specifically, print data to fill a page memory reduced to ⅛ its size otherwise can be transmitted in the example of FIG. 2A and print data to fill a page memory reduced to ¼ its size otherwise can be transmitted in the example of FIG. 2B. Subsequent to print data transmission, the data to render the image in a desired size and resolution can be obtained by increasing the information quantity or the number of pixels in the recovered image.
FIG. 3 is a flow chart illustrating a conventional resolution increasing method using a lookup table. As illustrated in FIG. 3, at operation S31 a binary image is input, and an image process at operation S32 is performed by accessing a lookup table (not illustrated) to retrieve image data corresponding to an input block of the input binary image. When the image process is performed on all input blocks constituting the binary image at operation S33, the resolution increasing method is terminated.
In such a conventional method, the image process is performed using one predetermined lookup table irrespective of characteristics of the input binary image. In the case where the lookup table contains mean values of pixels obtained from a representative training image, artifacts, such as blurring, may occur in a resultant image pattern after the image processing of a specific input image.